Selling trips to the moon? Sending astronauts to Mars and back? These sound like 1960s-era science fiction adventures, but they're actually in the works for later this decade. Will these privately backed projects get off the ground? That's the billion-dollar question.

The two ventures are the focus of Wednesday night's installment of "Virtually Speaking Science," a talk show that airs online via BlogTalkRadio with a live audience in the Second Life virtual world. I'm your host, and my guests are Taber MacCallum, Inspiration Mars' chief technology officer; and Doug Griffith, general counsel for Golden Spike.

The hour-long show starts at 9 p.m. ET (6 p.m. PT), but if you miss the live program, never fear: You can always download the podcast from BlogTalkRadio's archive or iTunes.

Both Golden Spike and Inspiration Mars are getting advice and moral support from NASA, but the financial support is coming from elsewhere. The lunar venture expects to bootstrap its way to profitability by selling its services — and initially through an Indiegogo crowdfunding campaign aimed at raising $240,000 (one dollar for each mile to the moon) by late April. So far, more than $7,500 has been contributed.

To Mars and backInspiration Mars is relying on seed money from California millionaire Dennis Tito, who became the first tourist to visit the International Space Station in 2001. Tito said his effort to send a spacecraft zooming past Mars during a favorable planetary alignment in 2018 is purely philanthropic, with the goal of inspiring future generations of Americans.

MacCallum, who took part in the Biosphere 2 experiment in 1991-1992 and went on to become a co-founder of Paragon Space Development Corp., said he's already noticed the inspirational effect.

"I keep hearing people say, 'This is the kind of thing America used to do, and maybe now we can do it again.' It's like we touched on a sore spot, and the reaction has been ... almost too positive," MacCallum said.

He said Tito's aim was merely to get some introductory exposure for the concept, in hopes that all the kinks can be worked out in time to make the 2018 deadline. Tito has committed to supporting the venture for its first two years, but he needs to raise the rest of the money for what's rumored to be a billion-dollar mission.

The team hasn't yet worked out the procedure for selecting the crew, but MacCallum said more than 100 applications have already been sent in — including some candidates with jaw-dropping credentials. "There are some where you say to yourself, 'Oh, my gosh!'" MacCallum told me. "Hey, listen, it's suddenly cool to be a middle-aged couple."

To the moonUnlike Inspiration Mars, Golden Spike is set up as a business, which will ultimately have to be supported by paying customers. The idea is to provide two-person trips to the moon for roughly the same cost as today's robotic missions to the moon. Golden Spike aims to do that by employing high-tech, low-cost hardware as well as a relatively low-risk mission architecture. The company plans to pre-position a lander in lunar orbit, and only then send the crew and their moon-and-back booster on a subsequent pair of launches.

"Before it even launches, we know that the lander is working," said Griffith, who is drawing upon years of experience in space and aviation law.

Griffith said Golden Spike will serve as the outer-space analog of, say, United Airlines, contracting with other companies for flight hardware. The company is working on design studies for launch vehicles, landers and other equipment. It's also talking with potential customers — and trying to convince the skeptics that it's really possible to put people on the moon, almost half a century after NASA did it in 1969.

"The consensus seems to be that it's doable within the prices we're talking about," Griffith said. "All of the skepticism seems to be about whether there are space agencies or billionaires who are willing to pay the price. That is the big unknown. ... I think we'll know in fairly short order whether the skeptics are right or wrong."

Griffith said Golden Spike's game plan calls for signing up its first customers for "right of first refusal" deals by the middle of the year, and getting its first flight contract by the end of this year.